The present invention relates to a DCxe2x80x94DC converter circuit, and in particular to a synchronous flyback converter circuit for operation in a continuous mode.
In DCxe2x80x94DC power supply of different kinds of electrical devices, power rectifiers are utilized in order to output a correct rectified output voltage. Typically a diode would be employed on the secondary side in order to obtain the rectified output voltage.
One way of obtaining a suitable rectifier circuit is to use flyback topology. In a flyback topology a primary side stores magnetic energy in a magnetisable core or the like during a charging interval. The energy is then fed to a secondary side during the so called flyback interval. The main advantage of a power rectifier circuit having a flyback topology compared to other rectifier circuits is its simple construction, which makes it cheap to manufacture.
Furthermore, flyback converters can be divided into two different kinds:
continuous mode flyback converters, and
discontinuous mode flyback converters.
In a continuous mode flyback converter the magnetic energy never drops to zero so that energy is continuously flowing either in or out of the core of the transformer, whereas in a discontinuous mode, intervals when energy is neither flowing in nor out of the core of the transformer occurs.
However, in a conventional flyback converter as seen in FIG. 1, which comprises, on the primary side, a primary winding 101 of a transformer 105 and a switch 103, and on the secondary side a secondary winding 107 of the transformer 105 connected to a diode 109 and an output capacitor 111 over which a load 113 can be connected, there is a problem associated with the voltage drop over the diode 109. Thus, in the case when the output voltage over the output capacitor 111 is low, e.g. less than 5 V, the voltage drop over the diode 109 becomes a significant part of the overall voltage, which makes the power converter inefficient for such low voltage applications.
Furthermore, U.S. Pat. No. 5,237,606 describes a power converter to be located in a remote terminal of a telephone system. The power converter is designed to be able to operate both in a continuous mode and in a discontinuous mode, which can occur in the specific application for which the power converter is designed. The power converter deals with the problem of a large voltage drop over the rectifying diode on the secondary side by means or using a FET transistor. However, since the aim is to provide a power converter which can operate both in a continuous mode and in a discontinuous mode, the circuit is not well suited for use in a continuous mode. This is due to the fact that in order to work properly in the discontinuous mode, control circuits are required, which besides being expensive, also galvanically interconnects the primary and secondary side of the power converter, thereby taking away the galvanic isolation between the primary and secondary side.
It is an object of the present invention to overcome the problems as outlined above and to provide a continuous mode flyback converter which has a simple construction, and yet being efficient compared to the converters according to the prior art.
This object and others are obtained by the power converters as set out in the appended claims.
Thus, by replacing the diode as conventionally used as a secondary switch in the secondary side of the DCxe2x80x94DC converter by a FET transistor a much lower voltage drop is achieved, which thus reduces the losses on the secondary side. The FET transistor is then directly connected to an auxiliary winding arranged in series with the secondary winding of the transformer.
Furthermore, by introducing a DC-blocking capacitor in a control arrangement on the secondary side the voltage provided by the auxiliary winding can be significantly reduced, thus reducing the driving losses.
Also, if a short-circuit of the output of the converter should occur sufficient voltage will still be available for turning the secondary switch on and off.